There are a number of different types of semiconductor-based imaging devices including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imaging devices include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
Solid state imaging devices include an array of pixel cells, which converts light energy received through an optical lens into electrical signals. Each pixel cell contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the array of photosensors are processed to render a digital image.
Imaging device pixel cells are sensitive to light in the visible spectrum. Naturally, however, the pixel cells used in digital imaging are essentially black and white (light and dark) images. To capture color images, the spectral components of incident light must be separated and collected. To this end, multiple band-pass color filters are imposed in front of the image sensor cells over the photosensitive areas of the cells. Color filters are typically pigmented or dyed material that will only pass a narrow band of visible light, e.g., red, blue, or green. For most low cost CMOS or CCD imaging devices, the color filters are integrated with the pixel cells in a patterned array. A common example of a color filter pattern is the tiled color filter array illustrated in U.S. Pat. No. 3,971,065, and commonly referred to as “the Bayer pattern” color filter. The color filters allow what would otherwise be black and white image sensors to produce color images.
As shown in FIG. 1, the Bayer pattern 15 is an array of repeating red (R), green (G), and blue (B) filters. Half of the filters in the Bayer pattern 15 are green, like green filter 3, while one quarter are red and the other quarter are blue. As shown, the pattern 15 repeats a row of alternating red and green color filters followed by a row of alternating blue and green color filters.
The Bayer patterned filters (or other patterns) may be deposited on top of an array of pixel cells 22 in the manner shown in FIG. 2. Specifically, an array of pixel cells 22 is formed in a semiconductor substrate 10. Each of the pixel cells 22 has a photosensitive element 12, which may be any photon-to-charge converting device, such as a photogate, photoconductor or photodiode. The color filter array 15 is typically formed over a metal layer 18 in the imager 20, separated from the photosensitive element 12 by insulating layers like an interlevel dielectric layer (ILD) 14 and a passivation layer 16. The metal layer 18 may be opaque and used to shield the area of the pixel cells 22 that is not intended to be light sensitive. Convex lenses 21 are formed over the color filter array 15. In operation, incident light is focused by the lenses 21 through the filter array 15 to the appropriate photosensitive element 12.
One problem associated with conventional color filter arrays is illustrated in FIG. 3. As shown, a color filter array 99 often has a non-uniform surface. This variation can be caused, for example, by the inherent non-uniformity caused by spin-coating the color filter array material on any surface of the imager 20. In fact, it can be seen in FIG. 3 that the surface unevenness occurs in two ways: (1) within an individual color filter and (2) among color filters within the array.
The first variation, i.e., variations within one color filter, is shown in FIG. 3 at pixel 51, which has a severe dip in the material used for the individual color filter. Specifically, the material has a greater thickness at the edges for this pixel than it does in the center. This type of variation in the color filter material surface is most common in the color filters formed during the latter part of the color filter array formation. Specifically, the first color filters formed on a flat surface can typically achieve sufficiently even surfaces; however, those filters formed after the first color filters are in place may exhibit unevenness due to the thickness variation achieved during reflow of the material. The variation of material thickness within any one color filter may be as great as 2000 Angstroms, depending on the particular characteristics of the color filter material.
Conventional color filter arrays 99 may also suffer from non-uniformity across the surface of the entire array (i.e., variation among pixel sensor cells within one die). This phenomenon is illustrated by dashed line 103 on FIG. 3, where the height of the color filters near the center of the array (HCA) may be greater than the height of the color filters near either outside end of the array (HEA). Within one die, this pixel-to-pixel variation can be caused by resist flow-over during processing of the color filter material causing the non-uniform distribution of color filter material. The across-array surface unevenness may have a variation of about 400-500 Angstroms of height differential from the center to sides of the array.
The surface unevenness can cause problems for an imaging device 20 (FIG. 2) having the uneven color filter array. For example, an uneven surface does not create a solid foundation for a microlens array, which is typically constructed over the color filters, and is less stable with an uneven foundation. In addition, an uneven color filter array can cause imaging efficiency reduction by creating additional fixed pattern noise or a shading effect. Specifically, fixed pattern noise, which is a spatial variation in pixel output values under uniform illumination, results from the variation of color filter material within one pixel cell. An undesirable shading effect occurs as the result of non-planarity of the color filter array over the entire surface of the array. Thus, having an color filter array with even topography can advantageously help to create a solid foundation for microlenses, reduce fixed pattern noise, and decrease undesirable shading of a reproduced image.
A conventional technique for dealing with the uneven topography of color filter arrays is to form an additional planarization layer over the surface of the color filter array, and use the flat planarization layer as a flat surface for the formation of microlenses. This additional planarization layer, however, adds to the stack height of the imaging device, disadvantageously resulting in additional space between the microlenses and the photosensor below.
In addition, known methods for providing even surfaces in semiconductor integrated circuit processing may not be effective for dealing with the uneven surface in a color filter array. Specifically, both chemical mechanical polishing (CMP) and etching techniques utilize reference surfaces to remove unwanted materials. For example, if etching were performed on the color filter array 99 of FIG. 3, the result would likely be that a uniform amount of thickness would be removed from each of the color filters, leaving the overall surface uneven. The lack of a planar reference surface makes these known techniques ineffective, and does not rectify the additional problems with uneven color filters discussed above.
Accordingly, a color filter array having an even surface and a simple and effective method of forming the same are desired.